1. Field of the Invention
The present invention relates to a coating structure formed on the outer surface of a metal pipe, and to a method for coating the surface of the metal.
2. Description of the Related Art
There has been a known anti-corrosive coating structure for metal pipes, as disclosed in Japanese Non-examined Patent Publication No.8-75084. In this anti-corrosive coating structure, a zinc plating layer is formed on the outer surface of a metal pipe, and on the zinc plating layer there is formed a coating layer comprised of a trivalent chromium compound. On the coating layer there is formed a resin layer such as polyvinyl fluoride layer, polyvinylidene fluoride layer, or epoxy resin layer. Metal pipes with such coating structure are resistant not only to chemical corrosion but also to chipping and impact. Because of such properties, the metal pipes having the coating structure have been used as pipelines for conveying a brake fluid and fuel in a motor vehicle.
In the above-mentioned coating structure, however, the coating layer is comprised of only a chromium compound. Therefore, the heat treatment conducted in order to bake a resin layer on a metal pipe causes thermal degradation to the coating layer, making the coating layer brittle. This brittleness, which is brought by the ouster of moisture from the coating layer, can cause defects (e.g., fine cracks). Moreover, when metal pipes with brittle coating layer undergo a secondary treatment involving plastic deformation such as termination process or bending process, the risk of causing more defects increases.
As described above, there is a problem that the coating layer with defects is prone to peel off from the plating layer, resulting in the resin layer formed on the coating layer being prone to peel off as well. For instance, when splashed pebbles crash on the metal pipe of a motor vehicle in driving, the impact of the crash can cause the resin layer to peel off. When the resin layer peels off, the coating layer is exposed at the outer surface of the metal pipe. In addition, when the coating layer has defects, substances which corrode the metal pipe (hereafter referred to as corrosive substances) can easily reach the outer surface of the metal pipe. Due to the above reason, the conventional coating structure can cause the metal pipe to rust, without exerting corrosion resistance (hereafter referred to as anti-corrosivity) of the coating layer.
The present invention was accomplished to solve the above-described problems, and an object thereof is to improve the coating structure of a metal pipe, specifically, to prevent the coating layer from becoming brittle by thermal degradation and to improve anti-corrosivity.
In order to achieve the above-described object, a feature of the present invention resides in a coating structure of a metal pipe comprising a zinc or zinc-nickel plating layer formed on the outer surface of a metal pipe and a coating layer predominantly composed of a chromium compound and a phosphoric acid compound and formed on the zinc or zinc-nickel plating layer.
In this case, applied to the metal pipe may be a single walled steel pipe made of an SPCC steel sheet, a single walled steel pipe or a double walled steel pipe made of an SPCC steel sheet which has been copper-plated in a predetermined layer thickness, a seam welded pipe, a solid-drawn steel pipe, and the like. The wall thickness of the metal pipe may be either thick or thin. The valence of a chromium compound comprising the coating layer is not limited; for instance, a chromium compound having a valence of 3, 6 or the like may be adopted.
Since the coating layer is predominantly composed of a chromium compound and a phosphoric acid compound, the above-described structure can prevent the coating layer becoming brittle by thermal degradation and can enhance anti-corrosivity of the coating layer. Therefore, the metal pipe subjected to secondary treatment requiring plastic deformation thereof can prevent from producing defects (e.g., fine cracks) on its coating layer. The prevention of the defects blocks the entry of corrosive substances to the outer surface of the metal pipe, enabling the coating layer sufficiently exerting its anti-corrosivity. Such coating layer whose anti-corrosivity is enhanced can protect the metal pipe from corrosion over a prolonged period.
Another feature of the present invention lies in that a resin layer is formed on the coating layer in order to protect the coating layer. In this case, the resin layer may be comprised of a polyvinyl fluoride layer or a polyvinylidene fluoride layer. The resin layer may also be comprised of an intermediate layer containing an epoxy resin formed on the coating layer and a polyvinyl fluoride layer or a polyvinylidene fluoride layer formed on the intermediate layer. Further, an outer resin layer containing polypropylene and polyethylene may be formed on the resin layer.
The resin layer may also be comprised of an intermediate layer containing an epoxy resin formed on the coating layer and a polyamide resin layer formed on the intermediate layer. On the resin layer, moreover, there may be formed an outer resin layer containing polypropylene and polyethylene.
In the above-mentioned structure, the resin layer is formed on the thermal-degradation-proof coating layer, securely contacted with the coating layer. Therefore, the resin layer protects the coating layer from corrosion, allowing the treated layer to protect the metal pipe from corrosion for a longer period of time.
In the other aspect of the present invention, a feature of the present invention resides in a method of coating a metal pipe which comprises the steps of forming a zinc or zinc-nickel plating layer on the outer surface of a metal pipe and forming a coating layer on the zinc or zinc-nickel plating layer by dipping the metal pipe with the zinc or zinc-nickel plating layer formed thereon in coating liquid containing chromium (a source of chromium) and at least one substance selected from the group consisting of oxyacid of phosphorus, oxysalt of phosphorus, anhydride of oxyacid of phosphorus, and anhydride of oxysalt of phosphorus.
Furthermore, another feature of the present invention resides in a method of coating a metal pipe which further comprises the step of forming a resin layer on the coating layer.
In this case, the resin layer may be any example selected from the following examples 1 to 6. The resin layer according to an example 1 is comprised of a polyvinyl fluoride layer or a polyvinylidene fluoride layer. The resin layer according to an example 2 is comprised of an intermediate layer containing an epoxy resin formed on the coating layer and a polyvinyl fluoride layer or a polyvinylidene fluoride layer formed on the intermediate layer. The resin layer according to an example 3 has an outer resin layer containing polypropylene and polyethylene formed on the polyvinyl fluoride layer or the polyvinylidene fluoride layer of the example 1. The resin layer according to an example 4 has an outer resin layer comprising polypropylene and polyethylene formed on the polyvinyl fluoride layer or the polyvinylidene fluoride layer of the example 2. The resin layer according to an example 5 is comprised of an intermediate layer containing an epoxy resin formed on the coating layer and a polyamide resin layer formed on the intermediate layer. The resin layer according to an example 6 has an outer resin layer containing polypropylene and polyethylene formed on the polyamide resin layer of the example 5.
The metal pipe having the above-described coating structure (or the metal pipe made by the above-described coating method) resists chemical corrosion, chipping, and impact, exerting excellent effectiveness when applied to a motor vehicle.